This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-094567, filed Mar. 30, 2000; and No. 2000-094875, filed Mar. 30, 2000, the entire contents of both of which are incorporated herein by reference.
The present invention relates to a display device and, more particularly, to a moving-film display device.
Recently, low power consumption is required in large display devices or in portable display devices. One display device which accomplishes this low power consumption is a moving-film display device which drives a moving film electrode by electrostatic force. Jpn. Pat. Appln. KOKAI Publication Nos. 8-271933 and 11-95693 disclosed moving-film display devices of this type.
As shown in FIG. 15, a moving-film display device has a pixel matrix, i.e., an array, defined by rows and columns of a plurality of pixels. As shown in FIG. 15, each pixel has a moving film electrode 11, a fixed portion 12, and a counter electrode 13. The moving film electrode 11 and the fixed portion 12 are connected to a signal line 41, and the counter electrode 13 is connected to an address line 42. The upper end portions of the moving film electrode 11 and the fixed portion 12 are colored in first and second different colors, e.g., black and white. The display color of each pixel is determined in accordance with whether or not the moving film electrode 11 bends by electrostatic force on the basis of a potential difference between the moving film electrode 11 and the counter electrode 13 (a potential difference between a signal potential and a counter potential).
As will be described later, the material of the moving film electrode 11 is so selected that the electrode 11 has hysteresis characteristics. Therefore, the moving film electrode 11 has stable states at positions where it is attracted to the fixed portion 12 and where it is attracted to the counter electrode 13, i.e., the moving film electrode 11 has bistability similarly to, e.g., a ferroelectric liquid crystal. This allows each pixel to display an image by driving the address line 42 for applying a voltage to the counter electrode 13 and driving the signal line 41 for applying a voltage to the moving film electrode 11 and the fixed portion 12.
A moving-film display device can also be driven by using a latch circuit 51 as shown in FIG. 16. That is, this latch circuit 51 with memory properties has first and second switches 52 and 53 which can be turned on and off. When the first switch 52 is turned on, a moving film electrode 11 and a fixed portion 12 are given a potential from a constant-potential line 54 having a predetermined potential. When the second switch 53 is turned on, the moving film electrode 11 and the fixed portion 12 are given a potential from a ground line 55. The constant-potential line 54 supplies a potential different from that of the ground line 55. Since a counter electrode 13 is given a potential from the ground, the moving film electrode 11 can be selectively bent by driving the latch circuit 51 of a corresponding pixel, thereby displaying an image.
Unfortunately, these conventional moving-film display devices have the following problems.
First, in the driving method using the simple matrix circuit shown in FIG. 15, when one pixel is selected and applied with a signal potential, the moving film electrode must bend to come in contact with the counter electrode or the bent moving film electrode must come in contact with the fixed portion before the next pixel is driven. For example, if a signal potential is applied to a second pixel connected to the same signal line as a first pixel before the moving film electrode of the first pixel finishes moving, this signal potential for the second pixel may cause the first pixel to behave in a way different from that obtained by the signal for the first pixel. After the moving film electrode comes in contact with either electrode, the signal is stably held because the moving film electrode has hysteresis characteristics. Accordingly, the drive time of one pixel must be longer than at least the time required to move the moving film electrode. This makes it impossible to realize a high-resolution display device or a display of motion images by shortening the time for driving one pixel.
The driving method using the latch circuit as shown in FIG. 16 requires one storage circuit for each pixel. Since this increases the number of constituent elements, the method cannot be realized at low cost. Additionally, since the structure is complicated by the use of one storage circuit for each pixel, fine pixels are difficult to form. Therefore, no small high-resolution display device can be realized.
A method of performing a gradation display in the moving-film display device will be described next. The basic operation of the moving-film display device is a binary display scheme having a state in which the moving film electrode bends and a state in which it does not. Hence, gradation display methods proposed so far are the following two methods.
The first method is a dither method which performs dot area modulation by forming one pixel from a plurality of elements, assuming that a set of the moving film electrode 11, the fixed portion 12, and the counter electrode 13 is one element. That is, one pixel is formed by n elements, and (n+1) gradation levels are displayed by turning on some of these elements.
The second method is a frame rate control (FRC) method which switches a display state and non-display state by dividing a time, during which an image is displayed once by supplying a signal to one pixel, into a plurality of units. That is, the time during which an image is displayed once by supplying a signal to one pixel is equally divided into n portions, and (n+1) gradation levels are displayed by turning on some of these portions.
Unfortunately, these gradation display methods have several problems.
In the dither method, one pixel is formed by a plurality of elements described above. Since, therefore, the size of one pixel cannot be unlimitedly decreased, a high-resolution display device is difficult to form. Also, even if small elements can be formed, the number of lines such as signal lines increases, and this makes the formation difficult.
In the FRC method, the time during which an image is displayed once by supplying a signal to one pixel is equally divided into n portions. Since this shortens the switching time, the signal frequency rises to make high-resolution images difficult to display. Additionally, when a large display device is formed, the wiring length increases, and this increases the possibility of occurrence of signal delays. High signal frequency of the FRC method is further problematic because the number of pixels also increases.
It is an object of the present invention to provide a moving-film display device having high resolution and capable of displaying motion images.
It is another object of the present invention to provide a display device capable of performing a gradation display even when high-resolution images are to be displayed or even when the display device is large.
According to a first aspect of the present invention, there is provided a moving-film display device comprising:
a pixel matrix defined by rows and columns of a plurality of pixels, each of the pixels comprising
first and second electrodes, one of the first and second electrodes being a moving film electrode capable of bending, at least its end portion having a colored portion, the other of the first and second electrodes being a counter electrode which opposes the moving film electrode, and
a switch connected to the first electrode;
a plurality of signal lines, each connected to the switches of pixels arranged in a raw in order to supply an image signal, for driving the first electrodes;
a signal line driver configured to selectively supply the image signal to the signal lines;
a plurality of counter potential lines, each connected to the second electrodes of pixels arranged in a column in order to give a counter potential to the second electrodes;
a plurality of address lines, each of address lines supplying a control signal to the switches for selecting the pixels; and
a controller configured to control the signal lines, the counter potential lines, and the switches;
wherein a display color of each pixel is determined when the moving film electrode bends by a potential difference between the moving film electrode and the counter electrode.
According to a second aspect of the present invention, there is provided a moving-film display device comprising a pixel matrix defined by rows and columns of a plurality of pixels disposed on an insulating substrate,
wherein, in each of the pixels, the device comprises:
a semiconductor switch disposed on the substrate and electrically connected to a signal line;
an intermediate conductor plate disposed on the substrate via a first insulating layer and electrically connected to the switch;
an upper conductor plate disposed on the intermediate conductor plate via a second insulating layer, the intermediate conductor plate and the upper conductor plate being electrically coupled with each other; and
a pair of electrodes including first and second electrodes which oppose each other while standing on the second insulating layer, the first electrode being electrically connected to the upper conductor plate, the second electrode being given a counter potential, one of the first and second electrodes being a moving film electrode which has a colored portion in an upper end portion and can bend, the other being a counter electrode which opposes the moving film electrode, and a display color of each pixel being determined when the moving film electrode bends by a potential difference between the moving film electrode and the counter electrode.
According to a third aspect of the present invention, there is provided a display device comprising:
a pixel matrix defined by rows and columns of a plurality of pixels, each of the pixels comprising a pair of electrodes including first and second electrodes opposing each other, and a colored portion which determines a display color of the pixel by changing an exposed state thereof in accordance with a potential difference between the pair of electrodes;
a plurality of signal lines which run along the pixels to give the first electrode a signal potential as an image signal;
a counter potential line disposed to give a counter potential to the second electrode;
a capacitor so disposed in each of the pixels as to connect a node between the signal line and the first electrode to a constant-potential portion different from the second electrode, in order to hold the signal potential given from the signal line;
a bypass formed in each of the pixels and including a resistor connected to the node in parallel with the capacitor in order to release electric charge from the capacitor;
a signal line driver configured to selectively supply the image signal to the signal lines; and
a controller configured to control the signal line driver, the controller applying a gradation display potential different from one pixel to another as the signal potential in order to perform a gradation display on the basis of an exposure/non-exposure time of the colored portion.